The mounting of tools concentric with the axis of rotation is necessary for good machining practice. If the tool is mounted in a machine collet eccentric to its center, several undesirable effects result. Cutting flutes at the high point of eccentricity will cut oversize. These same flutes will take a heavier cut than flutes at the low point of eccentricity and fail earlier. These same flutes will cut deeper than the other flutes, resulting in a rough micro-finish.
The mounting shanks of tools vary in diameter, but the tools must be centered in the collet regardless of tool shank diameter. However, it has been found that collets which clamp these tool shanks against sliding surfaces lack repeatability. This is because of variations in friction and also because of a slip-stick motion, both of which are characteristic of sliding surfaces.
Collets which are clamped by manual tightening or by the application of hydraulic or pneumatic pressure vary the circular runout since these methods are not exactly repeatable. For example, an operator may not tighten with the same pressure each time. As a result, the compliant members of the machine collet do not react the same and centering does not repeat.